Passive entry systems include a portable remote control and a base station. The remote control, for instance, a key fob (“fob”), is carried by a user. The base station is at a target. The fob and the base station wirelessly communicate with one another for remote control of the target.
Passive entry functions provided by a vehicular passive entry system include automatically unlocking vehicle doors when the fob is detected near the vehicle. The system may detect for the fob in response to a vehicle door handle being touched.
The location of the fob relative to the vehicle should be detected as the user in possession of the fob can unlock the vehicle by simply manipulating the door handle. The fob should be in close proximity to the vehicle before unlocking the vehicle. Otherwise, unauthorized users may be able to unlock the vehicle whenever the fob is within the general vicinity of the vehicle. Traditional systems have used the received signal strength of radio frequency (RF) and/or low frequency (LF) wireless signals from the fob to the vehicle to determine the location of the fob. However, these systems can be vulnerable to security attacks which relay or amplify the wireless signals.
A system that uses propagation delay (i.e., time of flight (ToF)) of RF wireless signals between the fob and the vehicle to detect the location of the fob addresses this vulnerability. The time delay of a two-way RF wireless signal exchange between the fob and the vehicle can be used to calculate the distance between the fob and the vehicle since the propagation speed of the signal is constant. The operation involves recording and communicating timestamps of the transmissions and receptions of the signal exchange between the fob and the vehicle.